The present invention is concerned with an improved apparatus for re-etching the shadow mask of a color cathode-ray tube.
Re-etching is a technique that has been adopted in the manufacture of color picture tubes featuring black-surround screens of the type described and claimed in U.S. Pat. No. 3,146,368 issued Aug. 24, 1964 in the name of Joseph P. Fiore and Sam H. Kaplan. Such a screen in its preferred form is a mosaic structure comprised of a multiplicity of phosphor dot triads disposed in an ordered array throughout the image or screen area. Each such triad includes a dot of green phosphor, a dot of blue and a dot of red phosphor. The screen differs from the conventional mosaic color screens by having phosphor dots reduced in dimension so that, instead of being in tangential contact with one another, they are separated from one another. Another particularly important and distinguishing feature is that the spaces which surround the phosphor dots are covered with a light-absorbing material, such as graphite. In short, each phosphor dot has a circumscribing ring of dark, light-absorbing material from which the screen derives its name "black-surround."
It is common practice in screening such a tube to locate and dimension deposits of the various phosphor materials by photographic printing in which a photosensitive material is exposed to actinic energy directed from a source to the screen through the apertures of a shadow mask assigned to that screen and positioned in operating relation thereto during the exposure step. In order to have phosphor dots of reduced size, while retaining the efficiency and tolerance characteristic of conventional triad color tubes, it is distinctly preferred that the electron beams of the tube be larger in diameter than the phosphor dots. This obviously presents problems in screening, namely, the problem of establishing desired relative dimensions of phosphor dots to excitation electron beams.
A most successful process of screening, achieving the desired relative dimensions, features the use of a shadow mask having a pattern of apertures initially dimensioned as required to screen the phosphor materials on the tube faceplate. After screening has been accomplished through well known photographic techniques, the mask is subjected to further etching to enlarge the apertures to a precisely controlled size in order that the electron beams, which obviously are dimensioned by the apertures of the mask through which the beams reach the screen, have an accurately determined diameter, larger than the phosphor dots. This general process is referred to as "etch-back" or "re-etch". A control system for precisely dimensioning the mask apertures in the re-etch process is fully described in U.S. Pat. No. 3,669,771, issued on June 13, 1972, in the name of Martin Lerner, and is assigned to the assignee of the present invention.
Re-etch apparatuses of the prior art have had the distinct disadvantage of only being able to properly etch a single size shadow mask. In a cathode-ray tube manufacturing facility where more than one size tube is produced this disadvantage causes many problems. In the usual case, such manufacturing facilities employ a conveyer system where each tube is processed seriatim. As a consequence, if re-etching is to be compatible with such a system a number of re-etch lines corresponding in number to the number of different tube sizes must be provided. Unfortunately, this involves extremely excessive capital investment which makes such employment prohibitive. The only alternative is to have one re-etch line which comprises re-etch apparatuses which may be converted over from time to time to suit different mask sizes. This however has the disadvantage of requiring stock piling of the shadow masks of each size, and storing the corresponding image screen panels. Such a procedure is expensive and disruptive to the normal flow of the overall manufacturing process. Also, as a consequence of stock piling, since a defect in the process for a particular tube size may not be detected until final test, a rather large number of defective tubes or panels may be manufactured before the defect is detected.
It is therefore an object of the present invention to provide an improved re-etch apparatus which avoids the disadvantages of prior art apparatuses.
It is another and particular object of the invention to provide a re-etch apparatus which is capable of re-etching shadow masks of different sizes.